Two Projects Extend HMT Findings on Atmospheric Rivers

July 15, 2010

Composite satellite image of total integrated water vapor from the Defense Meteorological Satellite Program's Special Sensor Microwave Imager for 16 February 2004. Warmer colors indicate enhanced water vapor content in the atmosphere. Atmospheric rivers are the narrow corridors of water vapor emanating mostly from the band of enhanced water vapor in the tropics. The white symbols show the locations of the two Global Positioning System (GPS) receiver networks, with meteorological sensors to allow for integrated water vapor measurements, being installed or upgraded as part of the projects described in the text. (Click image to enlarge)

Two projects led by the Earth System Research Laboratory (ESRL), the
National Weather Service (NWS), and the California Department of Water
Resources (CA-DWR) will help detect and monitor flood producing atmospheric
rivers (ARs), narrow regions of enhanced water vapor transport in
landfalling, midlatitude storms. First, CA-DWR is sponsoring a
Hydrometeorolgy Testbed (HMT) legacy project that includes upgrading 32
existing Global Positioning System (GPS) receivers in California with
meteorological data packages that will allow the receivers to measure
integrated water vapor, a key ingredient of ARs and the fuel for generating
precipitation. The existing GPS receivers are part of the Plate Boundary
Observatory operated by the National Science Foundation to precisely
monitor the position of the Earth's crust.

The second project is sponsored by NOAA's Coastal Storms Program (CSP) and
will fund the NWS Pacific Region to deploy GPS receivers at nine key
locations across the Pacific Basin, several of which will be within the
breeding grounds of ARs. Data from these sensors and improved modeling
through data assimilation will likely promote advanced detection of
developing oceanic storms and signs of their rapid development or
weakening. The data from all of the GPS sites in California and across the
Pacific will be made available to the public on the NOAA
GPS Meteorology Program web page
and to NWS operations through the Meteorological Assimilation Data Ingest System (MADIS).

In a maritime environment, a deep corridor of concentrated water vapor
transport is often found in landfalling storms. These corridors are
referred to as ARs because they tend to be quite narrow relative to their
length scale and because they are responsible for almost all of the
poleward water vapor transport. Consequently, ARs play a crucial role in
the global water cycle and represent a key phenomenon linking weather and
climate, yet until now, water vapor and its transport in ARs have not been
adequately monitored in space and time by other observations.

Floods cause more damage nationwide than any other type of natural
disaster. NOAA's HMT-West has provided an opportunity for NWS hydrologists
and meteorologists in California, Oregon and Washington to work closely
with NOAA researchers to develop and evaluate new methods of monitoring and
predicting ARs and their resulting impacts, including extreme precipitation
that leads to floods. This work is important given the general expectation
from climate models that extreme precipitation events will increase in a
changing climate. To help guide climate model diagnostics, ongoing research
in ESRL is focusing on explaining the origins of some of the strongest ARs,
which can connect to the tropics, and is exploring the role of the Sierra
Nevada's in modifying landfalling ARs. The GPS projects described here
represent a major step forward in regional climate-related research and
monitoring associated with the water cycle.